1. Technical Field
The present invention relates to a media guide mechanism, a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a mechanism for guiding a recording medium into a nip formed between a pair of rotary members, as well as a fixing device and an image forming apparatus incorporating such a media guide mechanism.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
FIGS. 1A and 1B are end-on, axial views of a roller-based fixing device 100 employed in electrophotographic image formation.
As shown in FIGS. 1A and 1B, the fixing device 100 includes a pair of rotary fixing members, one being a fuser roller 110 subjected to heating, and the other being a pressure roller 120 pressed against the fuser roller 110, which together form a heated area of contact called a fixing nip N therebetween, through which a recording sheet S is passed to fix a toner image under heat and pressure.
For proper conveyance of the recording sheet S, the fixing device 100 is equipped with a sheet guide mechanism 200 upstream from the fixing nip N, including a swivelable guide plate 210 along which the sheet S is guided into the fixing nip N. The guide plate 210 has its downstream end being free, and its another, upstream end hinged to a shaft 210a defining a rotational axis around which the plate 210 is rotatable.
An electrically controlled solenoid 220 is connected to the guide plate 210 to control position of the plate 210 around its rotational axis depending on the thickness of recording sheet S being guided. When supplied with an electrical signal changing according to the thickness of recording medium S detected by a sensor, the solenoid 220 causes the guide plate 210 to different operational positions with respect to an imaginary reference plane X in which the fixing nip N extends.
Specifically, as shown in FIG. 1A, where the recording medium in use is a relatively thin sheet S1, such as normal copy paper, the solenoid 220 moves the guide plate 210 to a first operational position in which the free end of the guide plate 210 is directed toward the fuser roller 110 away from the reference plane X.
The guide plate 210 thus establishing the first operational position directs the recording sheet 51 toward the fuser roller 110, so that the sheet 51 enters the fixing nip N with its leading edge passing between the roller surface and the downstream end of the guide plate 210, while temporarily bowing outward away from the reference plane X before entering the fixing nip N. Such temporary bowing of the sheet 51 transversely reinforces the sheet S1 to prevent creasing or other possible damage to the resulting print upon passage through the fixing nip N.
Conversely, as shown in FIG. 1B, where the recording medium in use is a relatively thick sheet S2, such as envelope paper or paperboard, the solenoid 220 moves the guide plate 210 to a second operational position in which the free end of the guide plate 210 aligns with the reference plane X.
The guide plate 210 thus establishing the second operational position directs the recording sheet S2 along the reference plane X, so that the sheet S2 directly enters the fixing nip N. Unlike the case with the relatively thin, flexible sheet 51, the recording sheet S2 does not bow outward away from the reference plane X before entering the fixing nip N, which would otherwise cause creases on the recording sheet S2, rather than prevent them, during entry into the fixing nip N.
Various similar techniques have been proposed to provide reliable media guide mechanism. For example, one known method proposes an image forming apparatus in which a solenoid adjusts position of a guide member according to several parameters, including physical properties of a recording medium, and environmental conditions under which printing is performed. Another method proposes a paper guide device that controls position of a guide plate using several types of manual or electric actuators, such as solenoid, cam, and linear stage.
Although generally successful for their intended purposes, those approaches employing a solenoid or actuator for controlling operational position of a guide member have several drawbacks.
One drawback is that the actuator-based position control requires a complicated electromechanical assembly, which is relatively large in size and is costly to implement. Another drawback is the relatively large size of the position controller imposes limitations on the design of the media guide mechanism, which often results in reduced serviceability or maintainability of the mechanism upon installation into an image forming apparatus.
For example, in the case of a solenoid-operated guide plate, provision of a solenoid and its associated driving circuitry adjacent to the guide plate restricts positioning of the shaft or rotational axis to the upstream end of the guide plate, making it difficult to remove the guide plate from the image forming apparatus due to the presence of bearings and other components for supporting the shaft at the upstream end of the guide plate, and due to the necessity of mechanically isolating the guide plate from the solenoid actuator at the downstream end of the guide plate.
Such limitations on the design of the media guide mechanism in turn limit the flexibility in the design and add to the overall size of the image forming apparatus, particularly where efficient positioning of unitized, removable components around the fixing device is required to prevent interference between the guide plate and the surrounding structure during removal from or installation in the image forming apparatus.